Satellite Shows Earth's Magnetic Field Bent During a Solar Storm

The solar system is awash in potentially harmful radiation from the Sun and cosmic sources, but the Earth faces it with a shield—its magnetic field, which traps and repels most particles, preventing them from piercing through to the ground.

However, when the Sun unleashes solar storms that slam into Earth, our planet’s magnetic field buckles and lets in more radiation. These particles can penetrate to lower latitudes during these storms, which can be hazardous to satellites, astronauts, and even airline flight crews that spend a long time on great circle routes near the poles. How far toward the equator these particles can go—what scientists call the geomagnetic cutoff—is an important indicator of how the magnetic field behaves during these storms.

Here Adriani et al. report observations during a geomagnetic storm on 14 December 2006. The measurements were taken by the Payload for Antimatter Matter Exploration and Light-Nuclei Astrophysics (PAMELA) instrument, built by an Italian-led European and Russian consortia and launched on a Russian Earth observation satellite. They represent the first time that scientists have been able to directly measure the cutoffs for protons with such high kinetic energies—from 80 MeV to several gigaelectron volts, or tens to thousands of times more energetic than typical solar wind particles.

From its pole-to-pole orbit, PAMELA was able to take detailed, repeated measurements, returning to the same region every 94 minutes to track the evolution of the storm. The researchers found that the changes in the geomagnetic cutoff are related to a complex stew of variables related to the configuration of the magnetic field. The best metrics included the total strength of the Sun’s magnetic field, the strength of the Sun’s magnetic field in the north–south direction, and the Kp index, which measures the strength of a geomagnetic storm.

The team also found that the magnetic cutoff latitudes changed by different amounts depending on the particles’ rigidity (momentum over electric charge). At the peak of the storm’s fury, particles that would normally be confined close to Earth’s polar circles could swoop as much as 7° lower. For particles with higher rigidity, the cutoff changed less, as they were already capable of encroaching much lower—deep into the subtropics. (Space Weather, doi:10.1002/2016SW001364, 2016)

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